Concentric Macro EMG


Technique

Recording surface

Recording area (mm2)

Number fibers in one motor unit studied (fibers)

Utility

Single fiber EMG

Small; 25 μm at side-port

0.0005

1–2

Analysis of neuromuscular transmission

Concentric EMG

Medium; 150 × 580 μm of beveled tip

0.07

10–20

Analysis of individual MUAPs

Monopolar EMG

Medium; 640 μm at tip

0.12–0.34

10–20

Analysis of individual MUAPs

MacroEMG

Large; un-insulated distal 15 mm of needle

Large

300–2000

Analysis of entire motor unit



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Fig. 13.1
Comparison of needles used for various types of electromyography studies. Line drawings illustrate, from top to bottom: single fiber, concentric, monopolar and macroEMG needles. The dark area corresponds to the recording area for each needle


The difference in these recording techniques is dependent largely upon the recording surface area of each needle. Single fibre EMG needles are characterized by a very small side-port that is 25 μm [1] in size and serves as the recording surface. This gives rise to a very small recording radius enabling potentials to be recorded from only 1–2 muscle fibers at a time. Conventional EMG utilizes needles with a larger recording surface area that may be either concentric or monopolar. Concentric needles are characterized by their beveled tips that contain a side port with a recording area of 150 × 580 μm [1]. Monopolar needles are characterized by their pointed tip with a recording surface area of 640 μm [1]. Of note, when concentric needles are used, a single ground surface electrode is required, while the use of a monopolar needle requires both ground and reference surface electrodes. MacroEMG needles are unique since they possess a recording surface area that is many times larger than these other needles. Although the needle contains a side-port similar to single fiber EMG, the distal 15 mm of the macroEMG needle is un-insulated, which gives rise to the larger recording surface area allowing it to perform analysis of all muscle fibers belonging to an individual motor unit (Table 13.1 [1, 2]).

Some of the traditional motor unit action potential (MUAP) measurement and analysis techniques described by Buchtal in the 1950s [3, 4] are time-consuming and pose significant challenges even in cooperative adult patients. Pediatric neurophysiologists are frequently called upon to differentiate normal versus abnormal neuromuscular pathology without the luxury of time and patient cooperation. In infants and younger children, the term “burst” EMG analysis is sometimes used to describe the sampling that occurs with a patient who is not able to follow commands consistently, in contrast to the traditional motor unit analysis technique used in older children, adolescents, and adults. Though experienced pediatric neurophysiologists are usually able to obtain accurate and valuable data from these smaller samplings, there are circumstances in which the amount of data collected is not optimal.

Compounding this difficult undertaking is that concentric needle electrodes record from about 10–20 muscle fibers of a motor unit at a time. While in adults one can overcome this limitation by moving the needle electrode inside the corridor of a motor unit’s territory to explore other areas, in children, especially infants and toddlers, this luxury of needle motor unit territory exploration is not always consistently attainable. Figure 13.2 shows a schematic representation of a concentric needle electrode inside the muscle, and the contribution of the nearby muscle fibers belonging to the same motor unit to the MUAP.

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Fig. 13.2
Schematic representation of a concentric needle electrode inside a normal muscle, showing the contribution to the MUAP of the nearby muscle fibers (simulated as white dots) belonging to a single motor unit. The semi circles (from closest to furthest from the needle electrode) approximate the muscle fibers that contribute to the MUAP amplitude, followed by those that contribute to its spike component, area, and lastly to its duration

Macro EMG as described by Stålberg [5] is a well-accepted technique for estimating the electrical size of a motor unit. It makes use of a large recording surface that makes it possible to sample the majority of muscle fibers of a motor unit, in contrast to the 10–20 muscle fibers sampled by the tip of a concentric needle electrode. As useful as the Stålberg technique is, a practical limitation is that it uses a single fiber needle electrode to record the cannula potential and does not afford a concomitant view of the concentric MUAP from which the Macro potential is derived. Another practical limitation is the significant cost associated with the purchase of macroEMG needles. To overcome these limitations, a technique known as concentric macro EMG or ConMac was developed by Jabre [6] that uses the cannula of a standard concentric needle electrode to record the concentric electrode’s cannula potential, referenced to a nearby surface electrode, allowing for a simultaneous view of the concentric MUAP and its corresponding macro potential [7]. To clearly distinguish between macro potentials derived from these two electrode types, we refer to the single fiber electrode cannula derived potential as the Macro potential, and to the concentric electrode cannula derived potential as the ConMac potential.

In this chapter, we will describe the ConMac EMG technique and its applicability to quickly and easily distinguish between normal, neurogenic, and myopathic motor unit changes.

In our experience, ConMac EMG is a fairly simple technique to learn and can be performed in under 5 minutes with a conventional concentric needle electrode using an EMG machine that allows for two channel recording.



Technical Description of the ConMac EMG Technique


The ConMac EMG technique requires a two-channel EMG machine, with one channel, typically channel 1, being used to record the concentric MUAP. This potential serves as a trigger to the cannula potential recorded on the other channel, in this case channel 2.

To extract the time-locked cannula potential on channel 2 from interfering MUAPs of nearby units, the channel 2 signal is averaged between 16 and 32 times, or until it remains stable and its amplitude and area can be measured easily. Filters used are 20–8 kHz for the concentric potential, and 8 Hz to 8 kHz for the ConMac potential. The technique and recording principle are shown in Fig. 13.3.

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Fig. 13.3
Concentric Macro EMG (ConMac) recording principle using a two-channel EMG machine. In the first channel (Ch 1), a concentric MUAP is recorded from the electrode’s active recording tip (a), with the cannula used as reference (r). This concentric MUAP is used to trigger a time-locked potential on the second channel (Ch 2), where the needle’s cannula is now the active (a) recording area, and a surface electrode nearby is used as reference (r). Because of the size and non-selective nature of the electrode’s cannula, there is much interference from nearby active motor units on channel 2. Interference can be greatly reduced by averaging this time-locked potential to generate the ConMac MUAP

ConMac action potentials look like a small motor response, very similar to an F-wave as shown in Fig. 13.4.

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Fig. 13.4
Typical concentric MUAP (bottom) used to trigger and average a ConMac MUAP (top) in a normal subject. Sweep speed: 10 ms/division. Sensitivity: Concentric MUAP 300 μV/division; ConMac MUAP 500 μV/division


ConMac EMG in Clinical Use


Our experience with the clinical use of ConMac EMG spans three decades. Several publications have explored the use of this technique in normal adults and in patients with neurogenic and myopathic pathology [8, 9].

These studies have shed some light not only on the motor unit territory and its architecture, but also on the validity of currently available MUAP normal values in the literature [10], and on the relationship between the cannula potential and the motor unit twitch force [11]. ConMac EMG has also been used for the demonstration of the motor unit recruitment size principle [12]. Finally, the shape of the ConMac MUAP remains relatively stable during needle manipulations, as shown by the scanning study in Fig. 13.5.

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Fig. 13.5
Scanning EMG. The concentric electrode is dragged through a motor unit territory’s corridor while recording its MUAP at different positions in the corridor (bottom), and trigger averaging the corresponding ConMac MUAP from these same positions in the corridor (top). Note the changes in the concentric MUAP morphology as the needle is moved through the corridor, and the stability of the corresponding ConMac MUAP’s morphology throughout the move. Sweep speed: 10 ms/division. Sensitivity: Concentric MUAP and ConMac MUAP 500 μV/division

This property of the ConMac technique makes it especially useful in children who typically do not tolerate the traditional sampling protocol that requires multiple movements of the needle electrode inside the muscle.


Normal Subjects


In normal adult subjects, cannula potentials usually have one or two phases, but occasionally may show more, such as in the tibialis anterior muscle [13]. Since the needle electrode’s cannula is non-selective, the ConMac MUAP is derived from the temporal and spatial summation of the majority of muscle fibers inside a motor unit [14]. Additionally, the ConMac potential’s area and amplitude are related to both the number and size of muscle fibers in the motor unit, a property that makes it especially sensitive to changes in muscle fiber size seen in neurogenic and myopathic pathology.

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Nov 18, 2017 | Posted by in PEDIATRICS | Comments Off on Concentric Macro EMG
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